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Tuesday, January 6, 2015

Late in 2030, Europe’s
Jupiter Icy moon Explorer (JUICE) spacecraft will twice zoom past Europa, a world
that has all the ingredients to harbor life.
During the minutes of each closest flyby, it will study an areas
identified from images taken in the 1990s by the Galileo spacecraft as
locations of recent geological activity.
Then after those two encounters, the spacecraft will move on to study
Jupiter and the moons Ganymede and Callisto.
Unless another space agency commits to another mission that will visit
Europa, this will be our only chance to explore Europa in the next several
decades. (But see the note at the end of
this post.)

Artist's concept of the JUICE spacecraft at Jupiter. Credit: ESA-AOES

Large space missions are
planned in progressively more detailed stages.
The European Space Agency selected the JUICE mission based on a concept
study (the so-called ‘Yellow Book’).
Last fall, the mission team completed the more detailed Definition Study
(the so-called ‘Red Book’) that tells us much more about how the mission will
study each of its target worlds.

Adequately describing the
exploration planned for the JUICE mission in the Definition Study would be too
much for one blog post. Over the coming
year, I’ll cover each of JUICE’s major scientific targets. Among the readers of my posts, however,
Europa has been a favorite subject, and I’ll kick off this series with the
plans to study this moon. (For anyone
willing to read a technical document, you can read the full JUICE Red Book here.)

Almost everything we know
about Europa comes from the 1990s Galileo spacecraft’s eleven flybys of this
moon. While the JUICE spacecraft will
perform just two flybys, it is likely to greatly deepen our understanding
Europa and may even radically transform it.

Much of Europa's surface shows evidence for recent geologic activity either in the form of surface features such as ridges, chaotic terrain, or reddish material that may be from the ocean below. Credit: NASA/JPL/University of Arizona

The Galileo spacecraft
carried instruments that were based on 1970s technology.(Problems with the Space Shuttle program
repeatedly delayed its launch, and it arrived at Jupiter in 1995.)The JUICE spacecraft will carry instruments
based on today’s technology.Where
Galileo and JUICE will have similar instruments, the JUICE instruments will be
far more capable.JUICE’s imaging spectrometer,
for example, will have approximately an order of magnitude more spectral sensitivity
and up to four times better spatial resolution than its Galileo
equivalent.JUICE will also carry a number
of new instruments that had no equivalent on Galileo, such as an ice
penetrating radar that will see structures beneath the surfaces of the three
icy moons.

JUICE also will return far
more data than its predecessor. Galileo’s
main antenna failed to deploy, leaving only the low gain antenna to return
data. Compared to the data return that
was planned, what we received from Galileo was like a getting a shot glass of
water instead of a lake. JUICE’s main
antenna will not require deployment, and minus a major mission failure, we’ll
finally get that lake-full of data along with many measurements the Galileo’s
instruments could not make.

One unfortunate fact
dominates all planning to study Europa.
Jupiter is surrounded by a radiation belt that grows more intense closer
to the giant planet. At Europa, the
radiation is more than 20 times greater than at the more distant Ganymede. This allows the designers of the JUICE
spacecraft to undertake comparatively modest radiation hardening to allow the
craft to safely spend months in orbit around Ganymede, but to plan for just two
quick dashes past Europa.

(As a side note, the
capabilities of the JUICE spacecraft and NASA’s proposed Europa Clipper, which
would flyby that moon 45 times, are roughly comparable in terms of scientific
payload, data communications rate, and power.
However, the estimated cost of the Clipper mission is almost twice that
of the JUICE mission, with much of the difference likely explained by the additional
radiation hardening needed to enable many Europa flybys.)

With just two flybys, the
JUICE mission will study in detail just a portion of Europa’s surface. The mission planners have chosen to focus
those flybys on areas where Galileo images revealed recent geologic
activity. These are terrains where the
icy shell has been broken to create regions of chaotic terrain and/or have
produced Europa’s characteristic ridges.
These regions also have surface materials that suggest water has been brought
to the surface from the underlying ocean.

To achieve the best
observing conditions for the instruments, the two flybys will occur within 15
degrees of longitude of the center of the far side of Europa from Jupiter and
will bring the spacecraft as close as 400 kilometers above the surface. Of the eight possible regions of interest
identified to date, seven lie on Europa’s trailing hemisphere which receives
the highest radiation loads. (Because
Jupiter’s magnetosphere rotates much faster than Europa travels around Jupiter,
the highly charged ions that create the high radiation levels slam into the
trailing hemisphere; the leading hemisphere has much lower radiation exposure.)

Examples of focus regions on Europa that the JUICE spacecraft may study. Red blocks are the regional focus regions, while the most detailed observations would occur directly below the path of the spacecraft (shown as the colored arcs). From ESA's JUICE Red Book.

For the two encounters,
the JUICE scientific team has identified three key goals. While the plan is to use almost all of
JUICE’s instruments during the encounters, for each objective only one or a few
of the instruments are expected to provide the prime measurements, while a few
others will provide secondary measurements.

Goal
1: Determine the composition of the non-ice material on the surface, with a
focus on substances that relate to potential habitability of the subsurface
ocean.

Galileo’s images revealed
that in many places, Europa’s surface ice was reddish or brown. Scientists believe these are locations where
fractures in the icy crust has brought materials from the ocean below to the
surface. Once on the surface, radiation
will modify these materials into sulphuric acid hydrates and hydrated salts. Further geologic activity may return these
modified materials to the ocean below where they may be important ingredients
to support any life.

JUICE’s instruments will observe
what materials are present on the surface, including those that could indicate
biological origins, and gather data to help to explain the origins of these
materials (whether they come directly from the subsurface ocean or have been modified
on the surface).

A suite of remote sensing
instruments will study the spectra of these materials to determine their
composition, with measurements ranging from the ultraviolet through the visible
and into the infrared portions of the spectrum.
The spacecraft will also use its in-situ particle instruments to
directly measure the composition of surface particles expelled from the surface
by radiation sputtering. For this goal,
the visible-Infrared imaging spectrometer (MAJIS) will be the prime instrument,
with the camera (JANUS), UV spectrometer (UVS), and the particle environment
package (PEP), which includes a mass spectrometer, playing supporting roles.

Goal
2: Search for liquid water below the surface.

For this goal, the ice
penetrating radar (RIME) is the star.
The radar’s radio waves will be able to penetrate as deep as nine
kilometers below the surface. From the
returned radio signals, scientists will detect both the subsurface structure of
the icy shell as well as detect bodies of water. They hope to detect either the top of the
ocean itself (assuming that the icy shell is thin, at least below areas of
recent geologic activity) or “lakes” trapped within between layers of ice.

Goal
3: Study the active processes

Europa has only a few
craters on its surface, allowing scientists to estimate that geologic processes
destroy and recreate the entire surface every 100 million years or so. Evidence for the processes that resurface
this moons lies across its surface in massive lines of ridges, locations where
the crust has been broken and rearranged like jigsaw puzzle pieces, and areas
of lumpy terrain. During the flybys, the
camera (JANUS) will take the starring role to image and map these terrains for
analysis by geologists looking for clues as to how these processes operate.

JUICE’s instruments also
will be able to directly ‘taste’ material form the surface. The JUICE spacecraft’s suite of in-situ
instruments (PEP) will sample the materials sputtered into space by this
continuous radiation bombardment to allow scientists to better understand how
this process remakes the surface material.

While the focus of the
Europa encounters will be on the high resolution measurements possible only in
the minutes around closest approach of each flyby, the spacecraft’s instruments
will also make regional observations Europa’s surface to map its geology and
composition in the hours before and after each encounter. In addition, JUICE’s camera and UV
spectrometers will be used to examine Europa for months from afar to search for
possible plumes of water. While there
hasn’t been a confirmation of the reported plumes observed with the Hubble
Telescope in 2013, any plumes present may only erupt irregularly or at low
intensities.

The JUICE spacecraft’s
modern instruments and working high gain antenna means it will return far more
data from Europa than the Galileo spacecraft could. Our understanding of this moon likely will
become much richer than it is today.

Detail of chaotic regions on Europa where geologic forces may have broken up the surface and released water from the ocean below. Credit: NASA/JPL/University of Arizona

However, it’s also
important to understand the limitations of the JUICE mission for studying
Europa. Only two locales will receive
high resolution imaging, and they will be within a single region of the
moon. The JUICE mission’s scientists
will select what they believe will be the two most interesting locations within
that region with regional studies as the spacecraft approaches and
departs. They will be relying on limited
and mostly low resolution Galileo data to select their targets that will then
be almost 25 years old. With just two
encounters, the JUICE spacecraft may give us a biased understanding of this
moon because the two sites may not be representative of the active regions
found across this moon.

The radiation hardening
needed to do a full survey of Europa just isn’t possible within this mission’s
budget (although I hope that the mission’s planners ultimately decide the
spacecraft can tolerate an additional flyby or two). Instead, the JUICE spacecraft will go on to
do a full study of Ganymede from orbit around that moon.

If NASA’s Europa Clipper
mission is funded and reaches Europa, it will conduct at least 45 flybys that
will be distributed across the globe and will study a wider variety of terrain
types. The larger number of encounters
give a much better chance of sampling all the important terrain types. With this many flybys and the possibility of
more during an extended mission, the Clipper can do repeat flybys of specific
locations to follow up on important discoveries made during the mission. A large number of globally-distributed flybys
also are required to enable certain studies such as the gravity studies of the
interior or magnetic induction studies of the ocean to constrain its volume.

The Clipper mission may
also carry a high resolution camera to search for safe landing sites for a
follow on lander mission. (This camera
was included in the last documents I saw, but the mission and its budget have
yet to be approved and a high resolution camera adds significant costs.) If the Clipper spacecraft carries this
camera, then it has a much better chance of finding a safe and scientifically
interesting landing site with forty-five encounters than the JUICE spacecraft
will have with two.

With luck, we will have
both the JUICE and Clipper missions.
JUICE will study Ganymede in detail as Clipper will do with Europa. In that case, the JUICE encounters will add
to the Clipper’s encounters and allow the measurements of the two missions to
be calibrated. If Clipper doesn’t fly,
then we will have two encounters for a regional study of Europa from a highly
capable spacecraft and its instrument suite.

Breaking news: I listened in on a presentation by Jim Green,
head of NASA’s Planetary Science Division at this week’s Small Bodies
Assessment Group. Dr. Green says that
NASA hopes that it will be able to use the $100M Congress added to NASA’s
budget for a Europa mission to enable a New Start for the Europa Clipper program. This is the term for when a mission goes from
the wish list to an approved program. This
is the first that I had heard that NASA’s management was looking to commit to a
Europa mission. This isn’t a done deal:
the President’s Office of Budget and Management (OMB) must also approve a new
start, and in the past they have not been.
(Congress must also approve a new start, but the substantial funding it
has already supplied suggests that it would.) We will see with the release of
the Fiscal Year 2016 budget request whether OMB’s stance has changed. IF a new start is given, then the important
questions will be the total budget for the mission and when launch is planned
(which might be in the mid-2020’s).

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.